Issue #147: Remove cmdstanr from remote

[athowes]

Description

This draft PR closes #147.

Checklist

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[athowes]

As discussed in F2F:

• This PR is making progress on the roadmap to the package being on CRAN
• Although now we don't have cmdstanr in depends, we do have it in suggests
• This means that we have core package functionality like epidist::epidist() which relies on a suggested package which is bad
• Possible route away from this is to switch backend = "cmdstanr" to either no default or another default (I think only available is "rstan")

@seabbs do you know your preference on route forward from here?

[seabbs]

I think make a new issue for handling this with my preference being to stop being opinionated about the backend in the core package. We can talk about doing so (and how to) in the vignettes. The minimal version is we use rstan everywhere and put a comment about preferring cmdstanr in the FAQ